1. Field of the Invention
The present invention relates to printed circuit boards with embedded capacitors, and manufacturing methods thereof. More specifically, the present invention is directed to a printed circuit board having embedded capacitors which is advantageous in terms of high capacitance, resulting from coating high-dielectric constant of a polymer capacitor paste on an inner layer of the printed circuit board and semi-drying the coated capacitor paste to a state of B-stage, and a method of manufacturing the same.
2. Description of the Prior Art
Although discrete chip resistors or discrete chip capacitors have been generally mounted on a printed circuit board (PCB), in recent years, PCBs having passive components such as resistors or capacitors embedded therein are under development.
Such embedded passive components technology for PCBs is set to embed passive components such as resistors or capacitors into the PCB by means of a new material and fabrication process, and thusly fabricated passive components can function as conventional chip resistors and chip capacitors. The embedded passive component PCB means that the passive components such as capacitors get buried in an outer layer or on an inner layer of the PCB. If capacitors as the passive components are integrally incorporated into the PCB regardless of the size of the board, it is called “embedded capacitor”. Such a PCB is referred to as an embedded capacitor PCB. The embedded capacitor PCB is characterized in that the capacitor is inherently placed as a part of the PCB, and thus it needs not be additionally mounted onto a surface of the board.
At present, techniques of fabricating the embedded capacitor PCB are classified into three types as follows.
First, there is provided a process of fabricating a polymer thick film (PTF) type capacitor by coating and thermally curing (drying) a polymer capacitor paste. The polymer capacitor paste is coated and then dried onto the inner layer of the PCB, on which a copper paste is printed and dried to define electrodes, thereby manufacturing the embedded capacitor.
Second, there is provided a process of fabricating an embedded discrete-type capacitor by coating a ceramic-filled photo-dielectric resin on the PCB, by Motorola, Inc., USA. The photo-dielectric resin containing ceramic powders are applied on the board, onto which a copper foil has been laminated to define each top and bottom electrode, after which circuit patterns are formed and the photo-dielectric resin is etched, to prepare the discrete type capacitor.
Third, there is provided a process of fabricating a capacitor by interposing a dielectric layer having capacitance properties into the PCB, capable of being substituted for a decoupling capacitor mounted onto the PCB, by Sanmina Corporation, USA. The dielectric layer comprising a power electrode and a ground electrode is laminated onto the inner layer of the PCB, to function as a power distributed decoupling capacitor.
Many efforts for developing various practicable processes and embodying such processes have been carried out, based on the three techniques mentioned above. However, the embedded capacitor PCB has not yet been actively fabricated. Therefore, standard technology for fabrication of the embedded capacitor PCB has not been established up to now, and a commercially available process technology is still under study.
A detailed description will be given of a printed circuit board having embedded capacitors and a manufacturing method thereof according to conventional techniques, with reference to the attached drawings, below.
In FIGS. 1a through 1e, there is sequentially illustrated a manufacturing process of a printed circuit board having embedded PTF-type capacitors according to a first embodiment of conventional techniques, in which the polymer capacitor paste is coated and dried (or cured) to prepare the PTF-type capacitor.
In detail, as shown in FIG. 1a, a copper foil formed of FR-4 is attached on an inner layer 42 of the PCB, and a dry film is coated on the copper foil, exposed and developed. Thereafter, the copper foil is etched to form copper foils 44a and 44b for positive electrodes, copper foils 43a and 43b for negative electrodes and clearance therebetween.
In FIG. 1b, capacitor pastes 45a and 45b comprising high-dielectric constant of ceramic powder-containing polymer are coated on the copper foils 43a and 43b for the negative electrode by use of a screen printing process, and then dried or cured. Herein, the screen printing process is performed by passing a medium such as an ink through a stencil screen with the use of a squeezing process to transfer patterns onto the surface of the board.
As such, the capacitor pastes 45a and 45b are coated on the copper foils 43a and 43b as well as the clearance between the copper foils 43a and 43b and the copper foils 44a and 44b for the positive electrode.
In FIG. 1c, a conductive paste, such as silver or copper, is formed onto positive electrodes 46a and 46b by means of a screen printing process, and then dried or cured.
In FIG. 1d, the capacitor layers formed on the inner layer 42 of the PCB as mentioned above are interposed between insulation layers 47a and 47b and then laminated. In the drawing, reference numerals 48a and 48b designate copper layers attached onto the insulation layers 47a and 47b. 
In FIG. 1e, through-holes (TH) and laser blinded via holes (LBVH) 49a and 49b are formed in the laminated layers, by which the capacitors on the inner layer of the board are connected to positive terminals 51a and 51b and negative terminals 50a and 50b of integrated circuit (IC) chips 52a and 52b mounted on an outer layer of the board, and thus play a part in embedded capacitors.
However, the above manufacturing process according to the first embodiment of conventional techniques suffers from cracking of the capacitor pastes 45a and 45b at end portions of the positive electrodes 46a and 46b. 
FIGS. 2a and 2b are views for explaining the problems of the PCBs manufactured by FIGS. 1a through 1e. 
When the capacitor pastes 45a and 45b are printed and dried on the copper foils 43a and 43b for negative electrodes as in FIG. 1b, cracks ‘C’ are generated as shown in FIG. 2a, due to the thickness of the coated copper foils 43a and 43b for negative electrodes. In case of being inserted into the PCB, the copper foils should be no more than ½ oz (18 μm) or 1 oz (36 μm) thick. However, the above printed capacitors have a thickness of 10 μm, and thus end portions of the copper foils 43a and 43b for negative electrodes become cracked. Thereby, when the copper pastes 45a and 45b connected to the copper foils 44a and 44b for positive electrodes are printed, short may occur between the positive electrodes and the negative electrodes.
In addition, the above manufacturing process is disadvantageous in terms of the non-uniform insulation distance between a first layer and a second layer shown in FIG. 1e. 
After the embedded capacitors 45a and 45b are laminated along with the insulation layers 47a and 47b according to the process shown in FIGS. 1a through 1e, insulation distance (A portion) between the first layer and the capacitor is considerably different from the insulation distance (B portion) between the first layer and the second layer, as shown in FIG. 2b. For instance, upon lamination of 80 μm thick insulation layer, the insulation distance between the first layer and the copper power electrodes 46a and 46b of the capacitor corresponding to the A portion is in the range of 20-30 μm. Meanwhile, the insulation distance between the first layer and the second layer FR-4 core 42 or the second layer copper foil corresponding to the B portion falls in the range of 60-70 μm. The reason for the large difference in insulation distance is that the capacitor pastes 45a and 45b are 10-15 μm thick and the copper pastes as the power electrodes 46a and 46b are 10-15 μm thick. Such a difference in the insulation distance between the first layer and the second layer results in poor impedance of signal circuit in the first and the second layers.
Further, the above process has the drawback, such as generation of a foreign substance by printing and drying of the capacitor pastes 45a and 45b and the copper pastes 46a and 46b. 
That is, when the capacitor pastes 45a and 45b are printed at a predetermined thickness of 10-15 μm and dried at 150° C. or higher for 30-90 min as in FIG. 1b, and then the copper pastes 46a and 46b are printed and dried thereupon, voids are frequently generated upon drying due to the foreign substance when being printed. Such voids cause layer short between the positive electrodes and the negative electrodes when the copper pastes 46a and 46b connected to the copper foils 44a and 44b for positive electrodes are printed and dried on top of the capacitor pastes 45a and 45b. 
Alternatively, referring to FIGS. 3a through 3f, there is sequentially shown a manufacturing process of a printed circuit board having embedded discrete capacitors according to a second embodiment of conventional techniques, in which a photo-dielectric resin is coated on the PCB to form the discrete capacitor. The ceramic filled photo-dielectric resin is coated on an inner layer of the PCB to prepare the embedded discrete capacitor, which is described in U.S. Pat. No. 6,349,456 patented by Motorola, Inc., the contents thereof being incorporated herein for reference.
In FIG. 3a, a ceramic powder-containing photo-dielectric resin 14 is coated on a PCB 10 having a conductive layer 12 attached thereon, and exposed and dried.
In FIG. 3b, a copper foil 16 is superimposed on the dried photo-dielectric resin 14. In the drawing, reference numeral 18 designates a sacrificial layer plated with tin formed on the copper foil 16 in order to be used as a copper etching resist.
In FIG. 3c, a dry film is laminated onto the sacrificial layer 18, and exposed and developed, whereby the sacrificial layer 18 and the copper foil 16 are etched to define top electrodes 20.
In FIG. 3d, the photo-dielectric resin 14 positioned under the top electrodes 20 is exposed and the photo-dielectric resin 22 is etched. As such, the top copper electrodes 20 are used as a photomask of the photo-dielectric resin 14.
In FIG. 3e, the copper foil 12 positioned under the etched photo-dielectric resin 22 is etched to define bottom electrodes 24.
In FIG. 3f, the capacitor layer 32 formed on the inner layer 10 of the PCB is interposed between insulation layers 26, onto which a metal layer 30 is layered.
Thereafter, the TH and the LBVH are formed in the laminated layers, and the capacitors 32 present in the PCB are connected to a power terminal and a ground terminal of IC chip mounted on an outer layer of the PCB, to manufacture a PCB having embedded discrete capacitors.
However, the above manufacturing process according to the second embodiment of conventional techniques is disadvantageous in terms of its high manufacturing cost.
In order to prepare the embedded discrete capacitor by the ceramic filled photo-dielectric resin 14 coated on the PCB, the top electrodes 20 and the bottom electrodes 24 should be patterned. By use of large numbers of processes, completely independent discrete capacitors are fabricated. Thus, cost required for fabrication of such capacitors increases. The photo-dielectric resin 14 is printed to the whole surface of the bottom electrode 12 as the copper layer and exposed, and the irradiated dielectric is etched and removed. Hence, when small numbers of the embedded capacitors are realized, large amounts of photo-dielectric positive resin are used, thus increasing loss of materials.
Further, the above process suffers from generation of short between the bottom electrodes 24.
That is, after the layered copper foil 16 is patterned to define the top electrodes 20, the photo-dielectric resin 14 remaining under the etched copper foil 16 is irradiated and then removed by use of an etching solution. As such, in case where a width of the portion to be removed is narrow, the dielectric resin 14 positioned above the lower copper foil 12 may not be etched. This is because the photosensitive agent in the photo-dielectric resin 14 is not completely irradiated, which can be attributed to thermal curing (110° C., 60 min) of the photo-dielectric resin 14. In particular, the dielectric resin 14 adjacent to the lower copper foil 12 is difficult to etch. Consequently, the lower copper foil 12 is not etched, and shorts between the bottom electrodes 24 may occur.
Turning now to FIGS. 4a through 4c, there is sequentially illustrated a manufacturing process for a printed circuit board having embedded capacitors according to a third embodiment of conventional techniques, in which a dielectric layer with capacitance properties is used. The dielectric layer having capacitance properties is inserted into the PCB, thereby fabricating the embedded capacitor capable of being substituted for a decoupling capacitor mounted externally on the PCB, which is described in U.S. Pat. Nos. 5,079,069, 5,261,153 and 5,800,575 patented by Sanmina Corporation, USA, the contents thereof being incorporated herein for reference.
In FIG. 4a, a copper coated laminate 61 having high-dielectric constant is disposed between copper layers 62 and 63, and a dry film is laminated on the copper layers 62 and 63. Then, through processes of exposure and development, the copper layers 62 and 63 are etched to define a power electrode of the capacitor and clearance between the electrodes.
In FIG. 4b, the inner layer 61 of the PCB subjected to the above step is interposed between insulation layers 64a and 64b and laminated, after which outer copper foils 65a and 65b are superimposed on the insulation layers 64a and 64b. 
In FIG. 4c, the TH and the LBVH are formed in the laminated layers, by which the capacitors present in the PCB are connected to a power terminal and a ground terminal of IC chips 68a and 68b mounted to an outer layer of the PCB to function as a power distributed decoupling capacitor. In the drawing, reference numerals 67a and 67b designate clearance between the ground electrode and the power electrode, which is widened to the extent of not bringing the TH or LBVH into contact with the electrodes.
However, in the above manufacturing process according to the third embodiment of conventional techniques, there is the problem of low capacitance, due to low dielectric constant of the embedded capacitor layer.
That is, in the case of using a thin dielectric layer with a thickness of 10-50 μm as in FIG. 4a, the material purchased from Sanmina Corporation, USA, comprises FR-4 dielectric materials having a thickness of 25 μm or 50 μm between the copper foils, used as the power electrode, and the ground electrode, in which the dielectric constant of FR-4 ranges from 4 to 5. Practically, capacitance per unit area (0.5-1 nF/in2) is considerably lower than generally used decoupling discrete chip capacitor (100 nF/in2). Thus limitations are imposed on fabrication of the embedded capacitor.
Further, in the above process, there is another problem, such as a thickened PCB due to insertion of the embedded capacitor.
That is, with the intention of obtaining high capacitance by means of the FR-4 dielectric, large numbers of embedded capacitor layers should be inserted, whereby the number of the layers of the PCB increases and the PCB becomes thick, thus increasing a manufacturing cost.
In addition, a further problem in the above manufacturing process is that, upon patterning the upper copper foil for use as a power electrode and the lower copper foil for use as a ground electrode, the thin dielectric film used to obtain high capacitance is subjected to the PCB fabrication process including lamination and patterning of the dry film, thereby causing shorts and cracks between the power electrode and the ground electrode, as shown in FIG. 5.
Specifically, FIG. 5 shows the problem of the PCB embedded with the capacitor formed by insertion of the dielectric having capacitance properties. As shown in FIG. 5, in the PCB having the embedded capacitor, 18-35 μm thick power electrode 92 and the ground electrode 93 formed on the 8-10 μm thick layer 91 with high-dielectric constant cause a short ‘G’ and cracking ‘F’.
In general, capacitance depends on the area and thickness of the capacitor, and is calculated from the following Equation 1:
Equation 1
  C  =            ɛ      r        ⁢                  ɛ        o            ⁡              (                  A          D                )            
wherein,
εr is a dielectric constant of the dielectric
εo is a constant having 8.855×10−8 
A is a surface area of the dielectric, and
D is a thickness of the dielectric.
In order to fabricate a capacitor having high capacitance, the dielectric constant of the dielectric should be high. Additionally, as the dielectric is thinned and surface area thereof is enlarged, the capacitance of the capacitor becomes increased. The capacitance of the bimodal polymer ceramic composite is 5-7 nF/cm2 while being 10 μm thick.
In U.S. Pat. No. 6,274,224 patented by 3M Co., there is disclosed the use of a thin film type dielectric layer having a thickness of 8-10 μm in a mixed composite form of BaTiO3 ceramic powders with a thermosetting plastic epoxy or polyimide between copper foils used as a power electrode and a ground electrode, in which capacitance per unit area (10 nF/in2) is relatively high, but shorts and cracking between the power electrode and the ground electrode may occur due to the thinness of the material.